Differential Diagnosis for a Diffuse, Severely Pruritic Rash

Single Most Likely Diagnosis

• Contact Dermatitis: This is the most likely diagnosis given the patient's history of cleaning brush from the woods while wearing minimal clothing, leading to exposure of large areas of skin to potential allergens or irritants. The diffuse, severely pruritic rash that appears on areas exposed during the activity supports this diagnosis.

Other Likely Diagnoses

• Allergic Reaction: Could be due to various substances the patient came into contact with during his outdoor activity, such as plants, insects, or other environmental allergens.
• Insect Bites or Stings: Given the outdoor setting, it's plausible the patient experienced multiple bites or stings from insects, leading to a pruritic rash.
• Photodermatitis: If the patient was also exposed to sunlight during his activity, photodermatitis (a condition where the skin reacts abnormally to sunlight) could be a consideration, especially if the rash appears in sun-exposed areas.

Do Not Miss Diagnoses

• Lyme Disease: Although less likely given the description of a diffuse rash, Lyme disease can cause a rash (erythema migrans) and is a critical diagnosis not to miss due to its potential long-term consequences if left untreated. The patient's recent outdoor activity in an area potentially endemic for Lyme disease warrants consideration.
• Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN): These are severe skin conditions usually triggered by medications or infections. While rare and not directly suggested by the history provided, they are life-threatening and should be considered in any patient presenting with a new, severe rash.

Rare Diagnoses

• Pityriasis Rosea: A skin condition characterized by a rash that can appear anywhere on the body, often starting with a single, large spot (herald patch) followed by smaller spots. It's less likely given the acute onset and severe pruritus but could be considered if other diagnoses are ruled out.
• Drug Eruption: If the patient has recently started any new medications, a drug eruption could be a possibility, although the history does not specifically suggest this.

Given the information provided, the most appropriate treatment among the options listed would likely involve symptomatic relief for the pruritus and inflammation, such as Diphenhydramine oral once daily as needed for itching and possibly a topical corticosteroid (not listed) for inflammation. Prednisone oral taper over 21 days could be considered for severe cases of contact dermatitis or allergic reactions to reduce inflammation. However, the specific choice of treatment would depend on confirming the diagnosis and assessing the severity of the condition.